These sprues become runways through which the metal flows and trapped air and gas escapes, allowing the bronze to flow to the bottom and also fill upward. 

There are several variables to consider, giving importance to proper ventilation and shrinkage as the molten bronze flows and distributes throughout the cavities of the mold.

The outcome of a good pour is critical, and depends largly on the science of gating and spruing.

INVESTING

"Investment" is the next step of building a rock-hard shell around the wax sculpture. Later in the process, when the wax has been melted out, the investment will serve as a mold for the molten bronze.

The ceramic shell technique begins by dipping the gated wax into vats of slurry followed immediately with a silica sand shower. This process builds a very thin wall of silica around the wax. 

When repeated approximately 9 or 10 times,  and allowing for drying time in between layers, a shell, about ½" thick, will form around the wax.

The grain sizes vary from very fine in the layer closest to the wax, to very coarse on the outside layer. This process was originally developed by the aerospace industry, and is the same ceramic used for the tiles on the space shuttles.

The wax is a "positive" which must disappear in order to create a cavity or "negative" for the bronze to fill, so the ceramic invested shells are fired in a kiln, the wax being melted or "lost" from the shell. Thus the phrase "lost wax casting" comes from this process. This will also harden the ceramic shell, making it suitable to accept the molten bronze.  

THE POUR